The present invention relates to a thrust pipe for laying pipe in the ground wherein the pipe is thrust axially through a prepared passage through the ground and particularly relates to the construction of the thrust pipe whereby advance of the pipe through the passage is facilitated.
Thrust pipes are old and well known and typically comprise a plurality of pipe segments arranged end-to-end with each segment having an outer pipe section which envelops or surrounds an inner pipe section. The outer pipe section is often formed of high-strength concrete and carries substantially the entirety of the axial compressive loads when the pipe segments are thrust forwardly in a axial direction along the passage. The outer pipe section carries a tool in advance of the pipe to loosen the ground in order to facilitate axial advance of the thrust pipe behind the tool. Thus, all axial compressive forces are transmitted from the tool through the outer pipe. This necessitates the formation of the outer pipe of high-strength materials. The inner pipe section conventionally serves as an internal liner for the outer pipe section and creates a seal against the escape of liquids or gases through the outer pipe section. Only very minimal compressive loadings are carried by the inner pipe.
The outer pipe sections of thrust pipes have also been formed of metal. However, it will be appreciated that the joints formed between adjacent outer metal pipe sections invariably require the formation of outwardly extending projections or shoulders in order to effect proper jointure of the adjacent pipe sections. Those projections or shoulders about the outer circumference of the pipe joints hinder and inhibit the axial advance of the pipe as the pipe passes through the surrounding soil of the passage. While it has been proposed to excavate a passage in front of the thrust pipe substantially diametrically larger than necessary to accommodate the diameter of the pipe, that leaves an annular cavity about the pipe when the latter is in place. As the cavity is filled by the surrounding soil, this settlement process causes subsidence of the ground surface above the pipe.
According to the present invention, thrust pipe is provided which is comprised of a plurality of pipe segments disposed in end-to-end axial relation one to the other wherein the pipe has substantial capacity for carrying compressive loads and a smooth, substantially continuous, outer surface without any outwardly protruding projections or shoulders which would inhibit the advance of the pipe through the passage. The present pipe is constructed to substantially fill the passage as it is advanced axially and enables the use of ductile cast pipes as thrust pipes. To accomplish that, the present invention provides, in contrast to the prior art discussed above, a plurality of pipe segments comprised of inner and outer pipe sections wherein the inner pipe sections are constructed of a material capable of carrying substantially the entirety of the axial compressive loading as the pipe segments are thrust forwardly and axially along the passage. By forming the inner pipe sections of such material, they may be advantageously formed of ductile, standard cast pipes with mutually supportive parts which project radially outwardly of the outer diameter of the inner pipe sections. For example, one end of each inner pipe section may be diametrically enlarged to telescopically receive the adjacent end of an inner pipe section whereby axial compressive forces may be readily transmitted in the axial direction from one pipe section to the next.
Each pipe segment also has an outer pipe section which terminates a distance short of one end of the corresponding inner pipe section. Each outer pipe section is axially spaced from the next adjacent opposite end of an adjacent outer pipe section. In this manner, when the pipe segments are assembled in end-to-end relation and axial thrust is applied, the compressive loading, as the pipe segments are advanced through the passage, is carried substantially entirely by the abutting inner pipe sections, while the outer pipe sections carry little or none of the compressive loadings due to the annular clearance spaces between the outer pipe sections. Stated somewhat differently, the present invention provides a reverse-type of construction than the prior art thrust pipe construction noted above in that the inner metallic pipe sections carry the compressive loading in the present invention, whereas the outer sections carry the compressive loading in the prior art.
Additionally, in accordance with the present invention, the outer surface of the outer pipe sections is of substantially constant diameter throughout the length of the pipe, and this facilitates axial advance of the pipe segments through the passage. That is, without any radially outwardly projecting protrusions or shoulders along the outer surface of the pipe segments, the passage through which the pipe segments pass axially may be accurately formed to the dimensions of the thrust pipe. In that manner, the thrust pipe completely fills the prepared passage as it is advanced axially and any previous need to form a larger cavity with consequent undesirable soil subsidence is eliminated. It is thus possible to use ductile cast pipes with mutually supporting, radially outwardly projecting parts at their joints, which provide a pressure-tight seal therebetween, while at the same time affording a substantially continuous, smooth outer surface to the outer pipe sections. Each outer pipe section has, of course, a larger diameter outer surface than the maximum diameter of the mutually supporting parts of the inner pipe sections at their joints. Thus, according to the present invention, it is possible to use ductile cast pipes as thrust pipes for laying by means of a thrusting process which has heretofore not been possible because of the projections of connecting parts for adjacent pipe sections. (Ductile cast pipes, known as ductile iron in the United States, are cast pipes which are treated by generating a certain directional position of the graphite particles within the cast iron.) In accordance with the present invention, each outer pipe section, therefore, can be made from lightweight concrete of limited strength inasmuch as it need not carry any of the compressive loadings resulting from axially advancing the pipe segments through the passage. Additionally, the present invention makes possible the formation of outer pipe sections from a foam plastic material. This may be particularly appropriate if thermal insulation is desired, for example, in the case of long-distance heating ducts.
An additional advantage of the present invention resides in the capability to use the form work or mold used to form the outer pipe sections as part of the pipe segments laid in the passage. That is, the mold or form used to form the foam plastic about the inner pipe section may be retained on the pipe segments so that, in use, the mold or form surrounds both the outer and inner pipe sections when the thrust pipe is disposed in the passage. Furthermore, the inner pipe sections may be covered with an anti-corrosion layer or the pipe sections per se may be formed from a corrosion-resistant metal.
Accordingly, in accordance with a preferred embodiment of the present invention, there is provided a thrust pipe for disposition in a passage in the ground by advance of the pipe axially through the passage comprising a plurality of thrust pipe segments disposed in end-to-end relation one to the other, each thrust pipe segment including an inner pipe section and an outer pipe section substantially enveloping the inner pipe section. Each of the inner pipe sections is formed of a material capable of carrying substantially the entirety of the axial compressive forces on the corresponding pipe segment as the pipe segments are thrust forwardly and axially along the passage. Additionally, each outer pipe segment has its outer pipe section terminating at one end short of the corresponding end of its inner pipe section. In a preferred form of the present invention, each inner pipe section has at one end a radially enlarged portion for telescopically receiving the opposite end of the adjacent inner pipe section whereby adjacent inner pipe sections may be sealed one to the other. With this configuration, the outer surfaces of the pipe segments are substantially the same diameter throughout their lengths whereby the pipe segments present a substantially constant diameter outline to the passage as the pipe is axially advanced within the passage.
In a further aspect of the present invention, the opposed adjacent ends of the outer pipe sections are spaced one from the other whereby axial transmission of compressive forces from one outer pipe section to the next outer pipe section as the pipe segments are thrust forwardly and advanced axially into the passage is substantially precluded.
In accordance with another aspect of the present invention, there is provided a thrust pipe for disposition in a passage in the ground by advance of the pipe axially through the passage comprising a thrust pipe segment including an inner pipe section and an outer pipe section about said inner pipe section, said inner pipe section being formed of a material capable of carrying substantially the entirety of the axial compressive forces on the corresponding pipe segment as the pipe segment is thrust axially along the passage, the pipe segment having its outer pipe section terminating at one end short of the corresponding end of its inner pipe section, the opposite end of said inner pipe section having a radially enlarged portion providing a radially enlarged interior pipe surface, the outer surface of said outer pipe section being of substantially constant diameter throughout its length whereby the pipe segment presents a substantially constant diameter outline to the passage as the pipe is axially advanced through the passage.
Accordingly, it is a primary object of the present invention to provide a novel and improved thrust pipe construction enabling the use of ductile cast pipe sections to be readily and easily laid in a passage without soil subsidence, wherein the inner pipe sections formed of the ductile cast material carry substantially the entirety of the compressive loading and the outer pipe sections covering the inner pipe sections form a substantially continuous, smooth surface throughout the length of the thrust pipe.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.